The present invention relates to technologies for enhancing the reliability of I/O (Input/Output) cells and in particular to a technology effective in stabilizing output signals in I/O cells with a parallel drive configuration in which two I/O cells are coupled in parallel.
In electronic systems, such as personal computers, digital cameras, and car navigation systems, such a volatile semiconductor memory as DRAM (Dynamic Random Access Memory) is widely used as a storage device.
In personal computers, for example, multiple volatile semiconductor memories are mounted over a board designated as memory module board. The read/write operation of the volatile semiconductor memories is controlled by a memory controller unit provided in a semiconductor device provided externally to the memory module board.
In a personal computer, for example, a memory module board mounted with multiple volatile semiconductor memories is coupled through a connector of a main board. The read/write operation of the volatile semiconductor memories is controlled by a memory controller unit provided in a semiconductor device mounted over this main board.
In digital cameras, car navigation systems, and the like, meanwhile, it is unnecessary to expand memories. Therefore, they are provided with a so-called on-board configuration and multiple volatile semiconductor memories and a memory controller unit for controlling the read/write operation of the volatile semiconductor memories are placed over an identical board.
As an I/O cell (I/O buffer) provided in a memory controller unit, so-called complementary I/O cell is known. In the complementary I/O cell, for example, one output signal is outputted as a complementary signal made up of a non-inverted signal and its inverted signal.
As this type of I/O cell, one in which the following is implemented is known: an input/output buffer with high driving capability, power supply cell accommodating to high currents, or the like is realized by causing multiple input/output cells to do output. (Refer to Patent Document 1, for example.)
[Patent Document 1]
    Japanese Unexamined Patent Publication No. Hei 6 (1994)-45566